It is known that one can determine the density of a gas from the velocity at which it flows through an opening, provided that the opening has a diameter which is relatively small by comparison to the mean free path of the gas molecules. In these systems, the effusion of a reference gas is compared with that of the gas to be analyzed. In accordance with Graham's Law, the square of the effusion velocity is inversely proportional to the gas density.
Using an effusometer of the Bunsen-Schilling type, reference gas and the gas to be analyzed are successively introduced or simultaneously introduced into one or two vessels which are pressurized. The duration over which the gas flows out of the vessel or vessels through the fine openings is determined.
From the results thus obtained, based upon thermodynamic and gas-kinetic considerations, the atomic or molecular weight and other parameters can be readily calculated.
The precision of the conventional effusion method is, however, unsatisfactory and the amount of material of the samples which must be handled is excessive for use of the technique and measurements in atomic physics.